If one flatmate paid for three, who owes whom?

When one flatmate pays Rs 3,000 for three, the other two each owe the payer Rs 1,000 and owe each other nothing. Say Sana is the flatmate who paid, for something you, Bilal and Sana share. Each share is Rs 1,000. You owe Sana Rs 1,000, and so does Bilal.

Said like that, nobody would argue. It gets harder once the bill is a line in a shared tab, because a tab keeps one number for each person, and that number does not say who it is owed to.

What does each person's number in a shared tab say?

Each person's number in a shared tab says how far ahead or behind they are against the whole group: what they paid, less their share of everything the group spent.

• Sana paid Rs 3,000 and her share is Rs 1,000. She is owed Rs 2,000.
• Bilal paid nothing and his share is Rs 1,000. He owes Rs 1,000.
• You paid nothing and your share is Rs 1,000. You owe Rs 1,000.

Every one of those numbers is a position against the group. It names nobody.

How does reading a shared tab go wrong?

Reading a shared tab goes wrong when you sit in your own seat and take the other two numbers as if each were about you. Sana is owed Rs 2,000, so you owe Sana Rs 2,000. Bilal owes Rs 1,000, so Bilal owes you Rs 1,000.

Both lines are wrong. You owe Sana Rs 1,000, and Bilal owes you nothing. His Rs 1,000 is Sana's.

Why does the mistake only start with three people?

With two people the mistake cannot happen, because there is nobody else for a number to be about. If only you and Sana had shared the Rs 3,000, she would be owed Rs 1,500 and you would owe Rs 1,500.

From three people up, a person's position and what they owe you are different numbers.

Who pays whom to settle it?

Two payments settle it: you pay Sana Rs 1,000, and Bilal pays Sana Rs 1,000. That is the fewest payments that leave everyone level.

So the line to read is your own: "You owe Sana Rs 1,000". Bilal's reads the same about him. Sana's two lines add up to the Rs 2,000 she is owed. Nothing runs between you and Bilal, so there is nothing to pay between you.

When is the flat square?

The flat is square only when every person's number is zero. Once you pay Sana and she agrees she got it, you are level, and the flat is not: Bilal still owes Sana Rs 1,000.

HisaabFlow, the money app you text, keeps this tab in a Room: everyone sees who owes whom, and it works out the fewest payments that leave everyone level. A payment moves a balance only after the person who received it taps Received. It is not open yet. See how Rooms work, or leave your email for early access and try the Guru today, before it opens.